A DFT-OFDM (discrete Fourier transform-orthogonal frequency division multiplexing) signal differs from a conventional OFDM (orthogonal frequency division multiplexing) signal in that DFT (discrete Fourier transform) transform is additionally performed before OFDM processing, and is currently used in an LTE (Long Term Evolution) mobile communications system and an IEEE 802.11 system. For example, the DFT-OFDM signal is used in uplink transmission in the LTE system. The DFT-OFDM signal is recorded in a standard as a DFT-S-OFDM (digital Fourier transform spread orthogonal frequency division multiplexing) signal, an SC-DFT-OFDM (single carrier digital Fourier transform orthogonal frequency division multiplexing) signal, or an SC-FDMA (single carrier frequency division multiple access) signal. In practice, the DFT-OFDM signal is usually referred to as a single carrier signal for short, and is one of candidate waveforms in a 5G mobile communications system. A baseband processing process of the single carrier signal is shown in FIG. 1. A DFT unit first performs M-order discrete Fourier transform DFT on M to-be-sent data symbols, such as QAM (quadrature amplitude modulation) symbols, and outputs M symbols obtained after the DFT to a mapping unit, the mapping unit maps the M symbols obtained after the DFT to a plurality of contiguous subcarriers, and then an OFDM unit performs OFDM processing on a plurality of mapped symbols, including IFFT (inverse fast Fourier transformation), parallel-to-serial conversion, cyclic prefix addition, and the like.
For the single carrier signal, DFT is additionally performed on the signal before the OFDM processing. Therefore, due to transform characteristics of DFT and IFFT, a peak-to-average power ratio (PAPR) of an output signal remains similar to a peak-to-average power ratio of the data symbol input to the DFT unit. In addition, in a wireless communications system, the QAM symbol is input to the DFT unit, and the QAM symbol maintains a relatively low PAPR. Therefore, a signal obtained after the DFT-OFDM processing still maintains a relatively low PAPR.
When there is only one carrier, because all allocated subcarriers are contiguous, it can be ensured that a signal output after the DFT-OFDM processing has a relatively low PAPR. However, as shown in FIG. 2, during aggregation of a plurality of carriers, especially when the plurality of carriers are non-contiguous on a spectrum, a signal output by using an existing DFT-OFDM signal processing process does not maintain a low PAPR, because after the foregoing single carrier signal processing technology is used for each carrier, a relatively high PAPR is generated when a plurality of single carrier signals output by the plurality of carriers are superimposed.
Likewise, on an LTE uplink, user equipment can occupy only contiguous physical resource blocks (PRB) to perform single-carrier waveform transmission, and only the contiguous physical resource blocks can ensure a low PAPR of a single-carrier waveform. Once a plurality of PRBs allocated to the user equipment are non-contiguous, a low PAPR of an existing LTE single-carrier waveform cannot be ensured. This is also an important reason why the LTE uplink does not support allocation of non-contiguous PRBs.
Therefore, how to maintain, during aggregation of a plurality of carriers or when a plurality of allocated PRBs are con-contiguous, a relatively low PAPR of a signal obtained after the DFT-OFDM processing is a key point for current research of the wireless communications system.